1. Field of the Invention
The present invention relates to a static convertor with AC terminals for connection to an AC voltage source and with DC terminals for connection to a load object with a variable electromotive force, said convertor comprising control members for controlling the direct voltage of the convertor between a maximum value and a mimimum value.
2. The Prior Art
The direct voltage U.sub.d.alpha. of a conventional, controlled convertor is obtained, in principle, from the expression EQU U.sub.d.alpha. = U.sub.d .multidot. cos .alpha.
where .alpha. is the control angle and U.sub.d the direct voltage obtained if .alpha. = 0. A maximum direct voltage in rectifier operation (0 .ltoreq. .alpha. &lt; 90.degree.) is obtained theoretically at .alpha. = 0, and maximum direct voltage in inverter opration (90.degree. &lt; .alpha. .ltoreq. 180.degree.) is obtained at .alpha. = 180.degree.. In practice, however, the control angle is always limited upwards as well as downwards in consideration of the commutation. The control angle .alpha. thus varies, in known convertors, between a minimum value .alpha..sub.min and a maximum value .alpha..sub.max. The maximum value .alpha..sub.max is determined by the necessity of ending the commutation process and of keeping the previously conducting rectifiers able to support offstate voltage at the zero passage of the commutating voltage at the latest, which occurs at .alpha. = 180.degree.. The time required for the commutating process increases with the load current of the convertor, and .alpha..sub.max must therefore be chosen so low that the maximum current of the convertor may be commutated with certainty. In typical convertors this means that .alpha..sub.max must be chosen relatively low, and in known convertors .alpha..sub.max lies, for example, at 140.degree.. This means that the maximally available direct voltage in inverter operation is considerably lower than U.sub.d.
In, for example, the typical application where a convertor is used for supplying and regulating a DC motor, a convertor must be dimensioned so that its maximum direct voltage in inverter operation (/U.sub.d . cos .alpha..sub.max /) exceeds the maximum electromotive force present in the motor. This is necessary if the motor current under all circumstances is to be able to be controlled down to zero. For given motor characteristics this means that a known convertor under practically all operational conditions will be considerably over-dimensioned, that is badly utilized.